Mitigating corrosion in oil well casing



April 1; 195s G. A. MARSH y 2,829,099

MITIGATING CORROSION IN OIL WELL CASING Filed Dec. 29, 1954 INVENTOR. v QJ ABY GLEN/V A. MARS/' ATTORNEY nite MITIGATNG CGRRGSION IN OIL WELL CASING Glenn A. Marsh, Crystal Lake, ill., assignor to The Pure Oil Company, Chicago, Ill., a corporation of Ohio My invention in general relates to means for mitigating corrosion. More particularly,y my invention pertains to improved means for providing cathodic protection of the internal surface of underground conduits, such as oil well casings, from corrosion.

It is Well recognized that where different metals of the electromotive series are placed in contact with each other and withV an electrolytic solution, such as brine or underground water, electrolytic currentflows betwen the two metals from the metal `which serves as the anode to the metal which serves as the cathode. This current is associated with the chemical oxidation of the anodic metal.

Metal conduits, such as oil well tubing and casings, have variations in composition from point to point such that cathode and anode combinations are formed at multiple loci along the metallic conduit surface. The corrosion due to such galvanic action at the internal surface of underground casings, conduits and the like is so extensive that it has become a major problem in the efficient operation of pipelines and wells.

The principle of cathodic protection has been developed as a means of mitigating or preventing corrosion in underground structures and the like. In one form, this system of protection may consist of the introduction of a more reactive sacrificial metallic anode in'close proximity to the conduit on the inside of the conduit. The metallic conduit along its entire inside surface is transformed from a series of the aforementioned cathodes and anodes, slightly differing in the composition of their metallic constituents from point to point, into a single cathode. By more reactive in this context is meant a metal of the electromotive series which when placed in contact with a less reactive metal of the same series becomes in relation to said latter metal an anode and the latter metal becomes a cathode. The anode may be defined as the electrode in a galvanic cell at which oxidation occurs. The cathode may be defined as the electrode in a galvanic cell at which reduction occurs. The introduced sacrificial anode undergoes corrosion, but this anode is readily replaced whenever necessary. The inside of the conduit, formerly subjected to corrosion at numerous points, is protected and preserved unaffected by the current flow.

A metal to serve satisfactorily as the .sacrificial anode not only must be less noble than the metal which is to be protected but should be relatively inexpensive and preferably easily fabricated into any desired form. Magnesium is preferred. Zinc may also be used but issomewhat less satisfactory, since the difference between its electrode potential and that of iron is smaller than the difference between the electrode potential of magnesium and that of iron.

It is an object of my invention to provide a suitable means of preventing corrosion of metal. f

Itis another object of my invention to provide a means for mitigating internal corrosion in oil well casings and other underground metal structures, such as conduits, pipelines and the like.

.t States Patent O It is a still further object of my invention to provide a means for employing cathodic protection of the internal surfaces of metallic underground structures in contact with an electrolyte.

It is yet a further object of my invention to provide self-clamping metallic rings which may be appropriately spaced in an oil well casing or other underground conduit as sacrificial anodes in a cathodic protection system to mitigate corrosion inside the casing or conduit.

In general, my invention comprises mitigating corrosion in an underground metallic structure by the use of sacrificial anodes of special design. More particularly, my invention comprises specially designed, self-clamping, anodic rings or cylinders which are appropriately spaced in an oil well casing, or similar underground metallic conduit, coming into. Contact with an electrolytic solution, such as underground water, so as to prevent or minimize corrosion. Where applicable, the self-clamping anodic'rings may be used on the outside of a pipe .or conduit.

A particular point of novelty in my invention is centered in the combination of a spring sheath with the anode to clamp it in position and coact to form a self-clamping anodic ring or cylinder.

Referring now tothe accompanying drawings for a more detailed description of my invention, Figure l is a cross-section of an oil well having therein spaced sacrilicial anodes. Figure 2 is afragmentary cross-section of a modification of the sacrificial anode taken on the line 3 3 of Fig. 6. Figure 3 is a fragmentary cross-section of another embodiment of the sacrificial anode taken on the line 3 -3 of Fig. 6, while Figure 6 is a plan view of the same embodiment. Figure 4 is a fragmentary crosssection of yet another embodiment of the anode, Figure 5 is a cross-section of a further embodiment of the anode, while Figure 7 is a plan View of another embodiment of the anode each of said sections being taken on the line 3 3 of Fig. 6.

In Figure l, pump l may be of any conventional type, casing-pump and is attached to sucker rod 2 which passes through the center of oil well 3 and is attached to a reciprocating device above the surface level. The oil well 3 passes through various strata and is defined by a metal casing 4 of a suitable thickness and ofany suitable metal, such as steel, which is more noble than the sacrificial ring-shaped or cylindrical-shaped anode 5. The metallic anodes 5 are spaced apart at predetermined distances from each other throughout the casing to provide a current density in the casing midway between the rings of 5-l0 milliamperes per sq. ft., a suitable current density to provide optimal cathodic protection in a metallic structure. Other suitable current densities may be obtained by regulating the number of anodes and the distances between the anodes. The sacrificial anodes 5 may be of any suitable metal. Magnesium is preferred although zinc may also be used. Magnesium is preferred because of its low cost, availability and the high difference in electrode potential between it and the iron or steel casing. Since magnesium has a high electrode potential in the electromotive series, it is effective as the anode despite the presence in the steel casing of alloying metals. The sacrificial anodes 5 are of any suitable shape commensurate with the shape of the well casing or conduit to be protected, and each is held in position in the casing by the outward pressure of an elastic, spring-like metallic or non-metallic sheath 6y having flanges .7 and S, which may be of a suitable metal, such as spring steel, or nonmetal, such as rubber, plastic, etc., and of any suitable thickness and design commensurate with the shape of the anodes and the well casing. The spring sheath forces the sacriiicial yanode outward against the casing wall 4 so that close contact is assured between the sacrificial anode 5 and said casing 4, and adequate cuthodic protection is afforded. Expansionand contraction of the anode and spring sheath are provided for by having a space interrupt the continuous wall of the anode and also that of the sheath; that is, the anode is split at one point, as is the sheath, the gap extending down through the anode and sheath from top to bottom and external to internal surfaces. Annuler space 9 between the inner surface of spring sheath 6, that is, the surface facing sucker rod 2, and the inner surface of the sacrificial anode 5 and the outer surface of said sucker rod 2, may be occupied by any suitable structure, such as bearings, packing and the like, or may bc left unoccupied by any structure. Said annular space may optionally be occupied by a structure of any suitable shape to provide adequate positioning of sucker rod 2 in the center of well 3. it will be noted that my invention is particularly applicable but not limited to the case where a pumping well is equipped with a casing pump and therefore has no tubing string.

In Figure 2, sacrificial anode 5a is held in position against the casing wall by flexible spring sheath 6a which is separated from said anode 5a by electrical insulating means 10.` Said electrical insulating means 1t) may be any suitable substance, such as wood, rubber, plastic or other insulating materials. The purpose of electrical in sulating means 10 is to prevent wasteful dissipation of energy from the sacrificial anode in protecting the spring sheath. As discussed above, spring sheath 6a may be metallic, such as spring steel or similar flexible metal, or may be made of plastic, rubber or other exible nonrnetallic substance. Where it is constructed of such nonmetallic substance, electrical insulating means 10 need not be present. Electrical insulating means i6 has flanges 11 and 12 which aid in clamping electrical insulating means 10 to sacrificial anode 5a and which are directed into the body of anode 5a. The elastic nature of said sheath 6a causes it to force sacrificial anode 5a tightly against the well casing, providing intimate contact between said well casing and abutting surface 13 of said anode 5a. [nner surface 14 of anode 5a and inner surface 1S of spring sheath 6a may be in communication with a suitable tubing or sucker-rod positioning means (not shown) as described in the discussion of Figure l, and serve to define the annular space between said tubing or sucker-rod (not shown) and said sacrificial anode-clamp structure.

inv Figures 3 and 6, anode 5b may be of any suitable metal as described above, and spring sheath 6b also may be of any suitable substance, metallic or non-metallic las described above. Electrical insulating means ma is also present and may be any suitable insulating material, such as described above. Said spring sheath 6b has leading edge 16 extending to a small degree over surface 13a of anode 5b adjacent the well casing, and is sharpened or pointed so as to engage and lightly penetrate or dent the well casing wall or conduit wall abutting said anode 5b to become firmly attached therein, thereby aiding the positioning of sacricial anode 5b and the intimate contacting of surface i3d of anode 5b with the abutting casing wall. Leading edge 16 may be continuous, but is preferably in the form of a plurality of spaced proiecting points as shown in Figure 6.

The sacrificial anode 5c, shown in Figure 4, is formed by casting it in a mold in which is properly positioned, spring sheath 6c having securing flange 17, internal holding structure l', and bottom securing edge i9. intimate contact between sacrificial anode surface 13b and the abutting well wall casing surface is obtained. This embodiment of the self-clamping anodes is similar to Figures l, 2 and 3 in structure and composition, except for the presence of the which is an extension of spring sheath 6c `and serves ns a means of positioning spring sheath 6c and securing it to uniquery designed, internal structure 18.

sacrificial anode 5c, so as to prevent or retard early crumbling and loss of large segments of anode 5c from the main body of anode during preferential corrosion of said sacrificial anode 5c. rl`hus, said internal structure ltfi tends to hold anode Sc together as said anode 5c becomes corrugated, pitted, and structurally weakened by corrosion due to the action of current. Anode 5c retains its unified structure for a longer period of time than if internal structure 13 or a similar internal structure were not present, thus extending the service life of said `anode 5c and reducing the danger of large fragments of said sacrificial anode 5c dropping off and proceeding down into the well.

In Figure 5, anode 5d is secured in position in the well or conduit by spring sheath 6d having internal anges ida imbedded in anode 5d. Intimate contact of surface 13e with the well casing surface and delayed segmentation and fracturing of anode 5d during corrosion of this modification by galvanic action are obtained. r[he embodiment in Figure 5 is similar to that in Figure 4 in that both structures have segments of the spring sheath internally disposed in the anode and arranged so as to inhibit early fracturing, break-off and loss of large portions of the anode during cathodic protection, and anodic destruction by galvanic action, in neither Figure 4 nor Figure 5 is there present an insulating barrier between the anode and sheath. Suitable modification of said anodic structures to incorporate insulating means to prevent dissipation of current by the spring sheaths is also contemplated.

As shown in Figure 6, sucker rod Z is supported in fixed position within said casing and well by fixed, positioning means 2d which may be bearing surfaces and the like located in annular space 9 between sucker rod 2 and the self-clamping anode structure. Said means 2t) may be, for example, a conventional type of packer which may be placed inside the anode to guide rod 2 in the center of the oil well. As shown at 21, sheath 6b and the underlying anode are split so that said sheath 6b and the anode may be expanded against the casing wall.

In Figure 7, the anode is placed on the outside of the conduit, casing or pipe rather than on the inside. The spring sheath 6e, which may be similar to that shown in Figure 3, overlies the upper and outer surfaces 0f the sacrificial anode instead of the upper and inner surface of the anode, Spring sheath 6e serves to compress the anode and force it tightly against the outer surface of casing, conduit or pipe 4, assuring intimate contact bctween anode and casing, conduit or pipe 4. Expansion and contraction of spring Sheath 6 and the anode are provided for by having space 21a extending, from top to bottom and internal to external surface, completely through a section of said sheath and the underlying anode. The modifications of the anode structure as shown in Figures l through 6 may be adapted for use on the outside of the well casing, pipe or conduit.

Upon examination of the above embodiments of my invention, it is seen that my invention involves a selfclamping anode structure which intimately contacts a conduit or a well casing surface, a plurality of which anodes may be spaced at distances to give maximum effective protection against corrosion of the conduit or well casing. As mentioned above, the current density of 5-10 milliamperes per sq. ft. midway between the separated rings appears to be the most desirable current density, and the anodes can be spaced apart so as to provide such current density. However, other current densities may `also be obtained. The distances depend on the nature of the electrolyte. As has been pointed out, self-clamping magnesium rings are the preferred anodes where steel or iron casings are to be cathodically protected, although zinc may be substituted for the magnesium in the rings.

My invention is not to be limited by the particular embodiments set forth in Figures l through 7. Thus, the metallic anode `itself may be abbreviated or elongated in a vertical direction so as to assume a ring-like appearance or a cylindrical tube-like appearance, depending on its vertical height. The thickness of each anode, that is, the distance from its surface which contacts the internal surface of the casing or conduit to its surface which abuts and defines the above-mentioned annular space surrounding the tubing-string in the Well may vary, depending on the amount of sacrilicial material deemed necessary to afford long-term cathodic protection. Each anode has a hollow central portion suitable for the passage of ka tubing-string orother suitable conduit therethrough and has a discontinuous wall.

The spring sheath may be of any shape which tends to expand the anode outward so as to hold it rmly against the inner surface of casing or conduit Wall, or tends to compress the anode to hold it firmly against the outer surface of theV casing or conduit under certain circumstances where outer surface protection is feasible.

Any suitable means may be utilized to effectively position the sacriiicial self-clamping anodes within the well casing, conduit'or pipeline. Thus, for example, a stack of the self-clamping anodes may be forced into an oil well casing by a gripping tool which compresses the spring sheath and thereby reduces the size of the anode, making it possible to quickly space the rings one by one throughout the oil well casing at optimal distances for ,full cathodic protection. Such a compressing tool may, for example, he of a design so that the top and bottom surfaces of the anode and sheath are gripped by movable jaws attached to a rod running through the center of the anode. The jaws after clamping tightly on the sheath and anode may exert an inward thrust to compress the sheath and the anode. Series of clamping jaws at levels along said rod hold a plurality of anodes, one above another in a compressed state. The rod may be lowered to the bottom of the oil Well casing and then each anode in turn moved up the casing to the correct position and released from the jaws of the rod and into intimate contact, on expansion, with the inner surface of the oil well casing. Similar means may be used in certain instances, where the outside of a conduit is accessible, to expand the spring sheaths and anodes to facilitate slipping anodes over the conduit, pipe or casing; after Correct positioning, the expanding means may be removed and the spring sheaths allowed to compress anodes tightly against the exterior surface of the casing, conduitor pipe.

The exact structural details of the sacrificial self-clamping anode operating as a cathodic protection device depend upon the particular conditions of the conduit or pipe, including the shape of the parts to be protected and the like. My invention is not limited to the use of the sacricial self-clamping anodes for cathodic protection of oil Well casing, but inds application and is intended for use wherever cathodic protection inside an underground A 6 conduit-like metallic structure is needed, or on the outside of a conduit wherever this method is applicable. Thus, oil pipelines, Water pipelines, gas mains, Water wells, gas wells, cable conduits and the like are, for example, included within the scope of my invention.

it is also contemplated to use the sacrificial, selfclamping anodes of my invention with such additional or supplementary equipment or means as are obvious to one skilled in the art.

i claim and distinctly point out:

l. The combination wit-h a metal conduit of a cathodic protection device for mitigating corrosion therein, said cathodic protection device comprising a sacrificial anode of a metal less noble thansaid conduit, said anode being annular in shape with an initial expanded form providing an outer surface slightly larger in diameter than the inside diameter of said metal conduit and having a single slot completely through the wall thereof permitting the same to be compressed to a size fitting within said conduit, an expansible split annular spring sheath of angular cross section iitted within said anode and having one wall portion contiguous with an annular portion of the inner Wall and another wall portion contiguous with an annular end wall of said anode, and said spring sheath expanding said anode to deforrny the same and effect `a tight contact with the inner surface of said metal conduit.V

2. A devicel in accordance with claim l in which adjacent surfaces of said expansiole sheath and said anode are separated by electrical insulation.

3. A device as defined in claim l in which said anode is formed of magnesium, said spring sheath has spaced prongs extending radially outward from the portion contiguous with the end wall of said anode for engaging the wall of the conduit to secure saidV anode in a fixed position therein, and electrical insulation is positioned between adjacent surfaccs of said anode and said sheath.

4. A device as defined in claim 1 in which said anode is formed of magnesium, said spring sheath is a split ring expansible steel member, and electrical insulation is positioned between and substantially covering the entire contiguous surfaces of said anode and said sheath and having fianges extending into the body of said anode.

5. A device as defined in claim l in which said anode is formed of magnesium and said spring sheath is a split ring expansible steel member having portions imbedded in said anode.

References Cited in the tile of this patent UNlTED STATES PATENTS 685,176 Ross Oct. 22, 1901 1,608,709 Milk Nov. 30, 1926 2,067,839 Godfrey Jan. l2, 1937 2,603,596 Vangsnes July 15, 1952 

1. THE COMBINATION WITH A METAL CONDUIT OF A CATHODIC PROTECTION DEVICE FOR MITIGATING CORROSION THEREIN, SAID CATHODIC PROTECTION DEVICE COMPRISING A SACRIFICIAL ANODE OF A METAL LESS NOBLE THAN SAID CONDUIT, SAID ANODE BEING ANNULAR IN SHAPE WITH AN INITIAL EXPANDED FORM PROVIDING AN OUTER SURFACE SLIGHTLY LARGER IN DIAMETER THAN THE INSIDE DIAMETER OF SAID METAL CONDUIT AND HAVING A SINGLE SLOT COMPLETELY THROUGH THE WALL THEREOF PERMITTING THE SAME TO BE COMPRESSED TO A SIZE FITTING WITHIN SAID CONDUIT, AN EXPANSIBLE SPLIT ANNULAR SPRING SHEATH OF ANGULAR CROSS SECTION FITTED WITHIN SAID ANODE AND HAVING ONE WALL PORTION CONTIGUOUS WITH AN ANNULAR PORTION OF THE INNER WALL AND ANOTHER WALL PORTION CONTIGUOUS WITH AN ANNULAR END WALL OF SAID ANODE, AND SAID SPRING SHEATH EXPANDING SAID ANODE TO DEFORM THE SAME AND EFFECT A TIGHT CONTACT WITH THE INNER SURFACE OF SAID METAL CONDUIT. 